KVM: arm64: pvtime: Fix potential loss of stolen time

We should only check current->sched_info.run_delay once when
updating stolen time. Otherwise there's a chance there could
be a change between checks that we miss (preemption disabling
comes after vcpu request checks).
Signed-off-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200804170604.42662-3-drjones@redhat.com

arch/arm64/kvm/pvtime.c

@@ -13,6 +13,7 @@
 void kvm_update_stolen_time(struct kvm_vcpu *vcpu)
 {
 	struct kvm *kvm = vcpu->kvm;
+	u64 last_steal = vcpu->arch.steal.last_steal;
 	u64 steal;
 	__le64 steal_le;
 	u64 offset;
@@ -24,8 +25,8 @@ void kvm_update_stolen_time(struct kvm_vcpu *vcpu)
 
 	/* Let's do the local bookkeeping */
 	steal = vcpu->arch.steal.steal;
-	steal += current->sched_info.run_delay - vcpu->arch.steal.last_steal;
-	vcpu->arch.steal.last_steal = current->sched_info.run_delay;
+	vcpu->arch.steal.last_steal = READ_ONCE(current->sched_info.run_delay);
+	steal += vcpu->arch.steal.last_steal - last_steal;
 	vcpu->arch.steal.steal = steal;
 
 	steal_le = cpu_to_le64(steal);